The Marsh Creek Watershed
Marsh Creek is the second largest Watershed in Contra Costa County. Marsh Creek originates 2,000 feet up the eastern slope of Mt. Diablo, approximately 35 miles east of San Francisco. It is one of the fastest urbanizing watersheds in California, and the creek flows for 30 miles through the rapidly growing communities of Brentwood, Oakley, and Antioch in eastern Contra Costa County and into the Sacramento-San Joaquin Delta. Marsh Creek forms an important ecological link between the Delta and the Diablo Range, and offers a vital natural refuge among multiplying subdivisions. Marsh Creek is also uniquely situated and provides significant spawning habitat near the mouth of the productive Sacramento-San Joaquin River Delta, so is therefore a priority watershed for restoration of salmon habitat in the Bay-Delta System.
Starting in headwaters in Morgan Territory on Mount Diablo, Marsh Creek meanders through various parks, public lands and private property and lands in the Marsh Creek Reservoir. Major tributaries are Briones, Dry, Deer and Sand Creeks. Briones Creek, which drains the undeveloped Briones Valley, flows into Marsh Creek at Marsh Creek Reservoir
The creek was dammed downstream from the where the Marsh Creek Springs used to be in the 1960s, forming Marsh Creek Reservoir. The dam is south of Brentwood and north of the intersection of Marsh Creek Road and Camino Diablo Road near the John Marsh House and the Marsh Creek State Park. The reservoir is actually a detention basin that holds back water during high rain events to help slow the flow of Marsh Creek through more populated and developed communities providing flood protection.
Marsh Creek below the Marsh Creek Reservoir was channelized in the 1950s and 60s when the area, an important agricultural area had incidents of severe flooding. The channelizeation of the lower watershed includes a drop structure near the city of Brentwood that at one time limited salmon migration. In 2010 a fish ladder structure was constructed at the drop structure that now allows salmon and other fish to access the upper parts of lower Marsh Creek. The lower stretch of the creek includes 11 miles (18 km) from the outfall of the Marsh Creek Reservoir into the western Delta at the Big Break Regional Shoreline area of the Sacramento-San Joaquin River Delta northeast of the city of Oakley. Dry, Deer, and Sand creeks all flow into Marsh Creek. Additionally Brentwood's Waste Water Treatment Plant discharges in to Marsh Creek after flowing over a water fall that oxygenates the water before entering the creek.